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You write the MIME and Content-type headers, then you overwrite them with the From and Reply-to headers. On that last line, you need to concatenate ( .= ), not assign ( = ).

(BTW, this is directly related to what we were working on in the other thread, so there's no need to worry about starting a new thread.
Also, generally speaking, you have to be pretty dang far off-topic before you get a warning about it in your own thread.
If you know it's unrelated, start a new thread; if it "might be" related (as in this case, where the issue cropped up right after changes you made), you're probably fine.)

As I described in post #2 above, on the last line of this snippet, you assign a new value to $from (using = ), where I would expect you to append to the current value (using .= ). That's why you're losing the Content-type headers, and your email is displaying as plain text.